Improvement in the process relating to alloys containing platinum group metals

ABSTRACT

METHOD OF DISPERSION STRENGTHENING RHODIUM-PLATINUM ALLOYS IN WHICH THE DISPERSED PHASE CONSISTS ESSENTIALLY OF AT LEAST ONE TRANSITION ELEMENT IN AN AMOUNT UP TO 20 ATOMIC PERCENT SELECTED FROM THE TITANIUM, VANADIUM, ZIRCONIUM, NIOBIUM, HAFNIUM AND TANTALUM.

United States Patent fimce 3,676,114 Patented July 11, 1972 3,676,114 IMPROVEMENT IN THE PROCESS RELATING TO ALLOYS CONTAINING PLATINUM GROUP METALS Gordon Leslie Selman, High Wycombe, England, assignor to Johnson, Matthey & Co., Limited No Drawing. Continuation-impart or application Ser. No. 740,017, June 26, 1968. This application Aug. 28, 1970, Ser. No. 67,986 Claims priority, application Great Britain, June 28, 1967, 29,958/67; Jan. 23, 1968, 3,520/68 Int. 'Cl. C22c /00 US. Cl. 75-172 7 Claims ABSTRACT OF THE DISCLOSURE Method of dispersion strengthening rhodium-platinum alloys in which the dispersed phase consists essentially of at least one transition element in an amount up to 20 atomic percent selected from the titanium, vanadium, zirconium, niobium, hafnium and tantalum.

This application is a continuation-in-part patent application of US. Pat. application Ser. No. 740,017, filed June 26, 1968 and now abandoned.

This invention relates to rhodium-platinum alloys.

One problem in the past with certain platinum group metal alloys, especially rhodium-platinum alloys containing 20 wt. percent or more of rhodium, has been their variable mechanical properties at high temperatures. This difliculty was formerly attributed to the presence in the alloys of dissolved oxygen, and efforts were made to overcome this by vacuum melting the alloy constituents together. Analysis of certain Rh-Pt alloys containing 25% Rh prepared by the vacuum melting technique showed that these alloys contained considerable quantities of additional ingredients picked up from the crucibles used in their preparation. Because of the difliculties experienced in obtaining sound ingots, the alloy melts were, in general, reduced with hydrogen prior to evacuation and then finally 'cast in an inert atmosphere.

In view of the foregoing We carried out systematic tests on these alloys to distinguish the efiects of dissolved oxygen from the effects of dissolved additional metallic ingredients on the mechanical properties, and in particular on the creep life of the alloys.

Some of the results of tests on the efiects of dissolved oxygen are shown in Table 1 below:

As will be seen from Table 1, it was found that for a 25% rhodium and platinum alloy prepared by argonarc-melting sintered compacts which had been previously degassed in a high vacuum, the creep life of the product when compared with that of the normal air melted prod not was considerably improved. Moreover, a 25% rhodium/platinum alloy, which had been vacuum melted in a zircon crucible, had an even better creep life.

These improvements in creep life are probably attributable to efiective degassing obtained by the argonarc-melting technique, especially since no hydrogen reduction was undertaken and where melting was carried out in a zircon crucible in vacuum, to the fact that very small quantities of zirconium were picked up from the crucible by the alloy.

In the Periodic Table, zirconium is located towards the left hand end of the range of transition elements in the second long period, whereas rhodium is located towards the right hand end. Similarly, platinum is at the right hand end of the range of transition elements in the third long period. This, together with the fact that we found that the platinum-rhodium alloy is significantly strengthened by the addition to it of a small amount of zirconium, led us to investigate the effect of alloying small quantities of one or more of those transition metals which lie to the left of chromium, molybdenum and tungsten in the 1st, 2nd and 3rd long periods respectively, with a platinum/ rhodium alloy. Our tests have indicated that platinum and rhodium and rhodium/platinum alloys so treated are strengthened. This strengthening is believed to be due to the tendency towards the formation of a phase of a hard, high-melting-point intermetallic compound such as Pt Zr or Rh Zr.

According to this invention there is provided rhodiumplatinum alloys consisting essentially of 9-45 wt. percent rhodium and from a trace up to 1 wt. percent of at least one transition element selected from the group consisting of titanium, vanadium, zirconium, niobium, hafnium and tantalum. Preferably rhodium is present in an amount between 20 and 40 wt. percent and the transition elements are selected from the group consisting of titanium, zirconium and niobium.

The alloys may in general contain up to 1 weight percent of the addition element(s), and are conveniently prepared by argon-arc-melting.

According to another aspect of the present invention, a metal alloy includes at least one addition element, as previously described, which has a tendency to form a stable compound with the metals rhodium and platinum. To establish the efiects of the inclusion of small amounts of addition elements, a series of tests was conducted and the results are shown in Table 2:

As will be seen from Table 2, a 25 rhodium/platinum alloy containing small additions of titanium and zirconium exhibits a still further improvement in creep life when compared with the same alloy containing no such addition (see Table 1).

Similarly the 20% Rh/Pt alloy containing small additions of niobium shows a marked improvement in creep life over the corresponding alloy containing no such addition.

Various methods of preparing alloys according to the invention will now be described by way of example.

Where small quantities of alloy are required, a green compact of the constituents is annealed in a vacuum and, thereafter melted in an argon arc.

Larger quantities of such alloys may be made by melting an appropriate amount of platinum or rhodium or both in, for example, a pure alumina crucible enclosed in a container and in an inert atomsphere; evacuating the container when melting is complete and, thereafter, simply introducing the addition element or elements to the melt. Where the transition element(s) to be added are too volatile, an inert atmosphere (for example argon) is introduced after the evacuation of the chamber and before the introduction of the element(s).

Finally, the alloy is cast.

Alternatively the alloys may be made without any possibility of contamination from the crucible material by melting in an argon arc furnace on a water-cooled copper hearth.

Alloys in accordance with the invention may also be produced by melting the constituents under reducing conditions in a crucible made from a material which tends to react very slightly, if at all, with the melt when heated under such reducing conditions. For example, the crucible may be made from zirconia. Melted in this way, under hydrogen for example, the rhodium/ platinum takes the addition element(s) into solution to produce the desired eflFect.

The following describes the production of batches of a 40% rhodium-platinum alloy with and without additives and the conditions under which those batches were tested.

A 200 oz. charge of 40% rhodium-platinum, made up from pressed sponge bars of rhodium and platinum, was melted in a zirconium silicate crucible by high frequency induction. When the alloy was fully molten, the whole charge was cast into a copper mould, producing an ingot 6" x 2%" x1%".

The ingot was hot forged in air at 1300 C. until its cross section was reduced to 1%." square and then hot rolled at 1250 C. to bar 0.32" square. The bar was then reduced to wire 0.040" diameter.

A length of the drawn wire was retained for test. The remainder was remelted, following the procedure outlined above, with the exception that 1% grams equivalent to 0.02% by weight of spectrographically pure niobium was plunged into the melt immediately prior to casting. The resulting ingot was again fabricated to 0.040 diameter by the methods described for the initial ingot.

Wires from the two batches of alloy were tested under tension at 1400 C. and 1400 p.s.i. and the following creep lives were recorded.

40% Rh/Pt-16 hours 40% Rh/Pt+0.02% Nb70 hours The creep life of the alloy was thus shown to be considerably increased by the addition of niobium.

Strengthened platinum and platinum group metal alloys, produced by the method according to this invention, are

particularly suitable for the fabrication of crucibles for glass making, bushings for glass fibre production and other glass handling equipment such as stirrers and weirs etc., and also heaters for furnaces and resistance heating wires, particularly those exposed to the air, as the presence of a thin layer of oxide on the wire surface will inhibit loss of weight due to the volatilisation of the platinum and/or rhodium metal present in the alloy.

What is claimed is:

1. A method of making a rhodium-platinum alloy containing from 9 to 45 wt. percent rhodium comprising melting the alloy constituents in a crucible under inert conditions, evacuating a container enclosing the crucible, adding up to 1 wt. percent of at least one transition element selected from the group consisting of titanium, vanadium, zirconium, niobium, hafnium and tantalum, and thereafter casting the alloy. 5

2. A method according to claim 1 wherein the alloy contains from 2040 wt. percent rhodium.

3. A method according to claim 2 wherein melting takes place in an argon atmosphere.

4. A method according to claim 1 modified in that the crucible is made from a material which tends to react very slightly, if at all, with the melt under reducing condi tions, and that melting is carried out under such conditions.

5. A method according to claim 1 modified in that melting is carried out in an argon-arc furnace on a watercooled copper hearth.

6. A method according to claim 1 modified in that melting is carried out in an induction furnace with a strong stirring action, introducing the transition element and, thereafter, chill coating the alloy so that the transition element constitutes a fine dispersion in the cast alloy.

7. A method according to claim 1 wherein the molten alloy is directionally solidified in a refractory mould.

References Cited UNITED STATES PATENTS 2,636,819 4/ 1953 Streicher -172 3,515,542 6/1970 Larsen 75-172 X L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner 

